Fluidized bed steam generation system and method of using recycled flue gases to assist in passing loopseal solids

Abstract: A fluidized bed steam generation system (10) and method in which recycled flue gases are used to assist in passing solid particulate material from the separator (22), through the loopseal (28), and back into the furnace (12). The solid particulate material separated in the cyclone separator (22) is assisted by recycled flue gases which have passed through the heat recovery section (26), an air heater (32), and a baghouse (38). The use of the recycled flue gases decreases the oxygen content which can cause oxidizing or burning of the solid particulate material which results in overheating or agglomeration of the loopseal.

Description: This invention relates to a fluidized bed steam generation system and a method of operating same and, more particularly, to such a system and method in which recycled flue gases are used to assist in passing separated solids from a separator section to a furnace section. Fluidized bed steam generation systems are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulphur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, the fluidized bed system offers an attractive combination of high heat release, high sulphur adsorption, low nitrogen oxide emissions, and high fuel flexibility. The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface. Other types of fluidized beds utilize a "circulating" fluidized bed. According to this technique, the fluidized bed density may be below that of a typical bubbling fluidized bed, the air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith. These circulating fluidized bed systems are characterized by relatively high solids recycling which makes the system insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore, stabilizing the emissions at a low level. The high solids recycling also improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle, and the resulting increase in sulphur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. In the event the reactor is in the form of a steam generator, the walls of the reactor are usually formed by a plurality of heat transfer tubes. The heat produced by combustion within the fluidized bed is transferred to a heat exchange medium, such as water, circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, which separates the water from the converted steam, which is routed either to a turbine to generate electricity or to a steam user. In these arrangements, the gaseous product from the furnace is often passed through a cyclone separator, which separates the entrained solid particulate material from the gaseous mixture and recycles the solid particulate material back into the furnace through a loopseal and a J-valve. The gaseous remainder from the cyclone separator is passed through a heat recovery section and to a baghouse in which the gases are drawn through bag filters using an induction draft fan to separate any remaining fine particulate material from the gases. In transporting the solid particulate material separated in the cyclone separator back into the furnace, air has been used to assist the movement of the material through the loopseal and into the furnace. An example of such an arrangement is shown US 4,419,965. However, when the recycled particulate material contains fine-size char it often combusts if air, which typically contains approximately 21% oxygen, is used to assist in passing the solid particulate material through the loopseal. The combustion raises the temperature of the material in the loopseal to relatively high levels. Further, when low grade fuels are used that contain a moderate to high amount of vanadium in their ash, low eutectic vanadium oxide compounds are formed when mixed with air. The combination of increased temperature caused by the combustion and the agglomeration of vanadium result in plugging of the loopseal which can cause shutdown and considerably reduce the efficiency of the system. According to the invention in one aspect there is provided a fluidized bed steam generation system comprising furnace for receiving a fluidized bed of particulate material including fuel and for combusting the fuel to form a mixture of entrained particulate material and gases, a separator for separating a first portion of the entrained particulate material from the gases, connecting means for connecting the separator with the furnace to pass the first portion of separated particulate material from the separator to the furnace, heat recovery means for passing the separated gases in contact with a cooling media, and a baghouse for receiving the gases from the separator means and for separating a second portion of particulate material from the gases, characterised by an air heater disposed downstream of the heat recovery means and upstream of the baghouse for adding relatively cool air to the cooled separated gases, and means for passing at least a portion of the separated gases back into the connecting means to assist in passing the first portion of separated particulate material through the connecting means and back into the furnace. According to the invention in another aspect there is provided a method of generating steam comprising fluidizing a bed of particulate material including fuel in a furnace, combusting the fluidized particulate material in the furnace to form a mixture of entrained particulate material and gases, separating a first portion of the entrained particulate material from the gases, passing that first portion of separated particulate material back to the furnace, separating a second portion of particulate material from the gases, and cooling the gases after the first step of separating and before the second step of separating, characterised by adding relatively cool air to the cooled gases before the second step of separating, and by passing at least a portion of the separated gases to the first portion of separated particulate material to assist in passing the first portion of separated particulate material back to the furnace section. In this way there is a decrease in the oxygen content of the gases used to assist in passing the solids through the loopseal. Therefore overheating of the solid particulate material in the loopseal is prevented and so thus avoids agglomeration of the solid particulate material in the loopseal. Also this results in an increase in the overall efficiency of the steam generation system. In a preferred embodiment of the invention the solid particulate material is recycled to the furnace section through a loopseal and a J-valve. The cool air is preferably added by a force draft fan and heated by the gaseous material. The invention will be more fully described, by way of example, with reference to the accompanying drawing which is a schematic representation depicting the system of the present invention. Referring to the drawing, a fluidized bed steam generation system 10 of the present invention, includes a furnace section 12 formed, in part, by an upright enclosure 12a. An air distributor, or grate 14, extends across the lower end of the enclosure 12a to define an air plenum 12b beneath the air distributor 14 for directing pressurized air from a source (not shown) through the air distributor 14 and upwardly through the enclosure 12a. A bed of particulate material 16 is supported on the air distributor 14 and extends the entire height of the enclosure 12a. The density of the particulate material in the enclosure 12a decreases as the distance from the air distributor 14 increases. A feeder inlet opening 12c and a recycle inlet opening 12d are provided through the walls of the enclosure 12a to allow particulate material to be introduced into the bed 16. The feeder inlet opening 12c is connected to and registers with a distributor pipe 18, through which new material is introduced to the bed 16. The introduction of recycled material through the recycle inlet opening 12d will be described. It is understood that the walls of the enclosure 12a are formed by a plurality of vertically-disposed tubes interconnected by vertically elongated bars or fins to form a substantially rectangular, contiguous, and air-tight structure. Flow circuitry (not shown) is provided to pass water through the tubes to convert the water to steam. Since this type of structure is conventional, it is not shown in the drawings nor will it be described in any further detail. An opening 12e formed in the upper portion of the enclosure 12a by bending back some of the tubes (not shown) forming a wall of the enclosure 12a. A duct 20 connects the opening 12e with a cyclone separator 22 disposed adjacent the enclosure 12a. The cyclone separator 22 includes an inner barrel 22a provided in its upper portion 22 to define an annular chamber 22b. A hopper 23 is positioned below the separator 22 and is connected to, and is integral with, the walls of the separator 22. The inner barrel 22a is connected, by a duct 24, with a heat recovery section 26 disposed adjacent the separator 22. A loopseal 28 connects the lower portion of the hopper 23 with the furnace section 12, through the recycle inlet opening 12d. The loopseal 28 contains a J-valve 28a for preventing the backflow of solids and/or gases directly from the furnace section 12 to the separator 22. The heat recovery section 26 has an opening 26a formed in its upper wall portion which receives the gases from the duct 24. The heat recovery section 26 is of conventional construction for transferring heat from the hot gases to a cooling fluid, such as water, which passes through heat exchange tubes, or the like (not shown), provided in the heat recovery section 26 and connected in the same flow circuit as the walls of the enclosure 12a. A gas flow duct 30 is formed adjacent the heat recovery section 26 for receiving the gases from the heat recovery section 26 and introducing the gases to an air heater 32 disposed adjacent the heat recovery section 26. A forced draft fan 34 is connected to, and in fluid communication with, the air heater 32 for introducing relatively cool air into the air heater 32. The cool air is mixed with the relatively hot gases passing through the air heater 32. The gases, now a mixture of air and gas, discharge from the air heater 32 into a duct 36 for directing the gases to a baghouse 38 disposed adjacent to the air heater 32. The baghouse 38 is of conventional construction and contains fabric filters (not shown) for providing a final separation of very fine solid particles from the gases received from the air heater 32. An induced draft fan (not shown) is connected to an outlet duct 40 extending from the baghouse 38 for drawing the gases through the fabric filters into the duct 40. A branch duct 42 is connected to, and in fluid communication with, the outlet duct 40 to direct a portion of the clean gases back to the loopseal 28 for assisting in the passing of the solids through the loopseal 28, and the outlet duct 40 directs the remaining portion of the clean gases to an external source (not shown). A forced draft fan 44 is connected to, registers with, and forces the recycled air from, the branch duct 42 into two ducts 46 and 48, which are connected to, and register with, the loopseal 28. A pair of hopper sections 50a and 50b are connected to the lower portion of the baghouse 38 for receiving the fine solid particles from the baghouse 38 and directing the separated or filtered solid material to a waste area (not shown). In operation, particulate fuel material and adsorbent material, as needed, are introduced into the enclosure 12a from feeders or the like (not shown) through the distributor pipe 18 and the feeder inlet opening 12c. Pressurized air from an external source passes into and through the air plenum 12b, through the air distributor 14, and into the bed of particulate material 16 in the enclosure 12a to fluidize the particulate material. A lightoff burner (not shown), or the like, is fired to ignite the particulate fuel material. When the temperature of the material reaches an acceptably high level, additional fuel from the feeder is discharged into the enclosure 12a through the distributor pipe 18 and the feeder inlet opening 12c. The material in the enclosure 12a is self-combusted by the heat in the furnace section 12 and the mixture of air and gaseous products of combustion passes upwardly through the enclosure 12a and entrain, or elutriate, the particulate material in the enclosure 12a. The velocity of the air introduced into the air plenum 12b, which passes through the air distributor 14 and into the interior of the enclosure 12a is controlled in accordance with the size of the particulate material in the enclosure 12a so that a circulating fluidized bed is formed, i.e. the particulate material is fluidized to an extent that substantial entrainment or elutriation of the particulate material in the bed is achieved. Thus, the gaseous mixture passing into the upper portion of the enclosure 12a is substantially saturated with the particulate material, and the gaseous mixture thus formed exits through the duct 20, and passes into the cyclone separator 22. In the separator 22, the gaseous mixture circles the inner barrel 22a in the annular chamber 22b and a portion of the entrained solid particulate material is separated from the gases by centrifugal forces. The solid particulate material falls into the hopper 23 and passes, via the loopseal 28, back into the enclosure 12a through the recycle inlet opening 12d where it mixes with the particulate material in the furnace section 12. The gases from the separator 22 pass upwardly through the inner barrel 22a and pass to the heat recovery section 26, via the duct 24. Heat is removed from the gases as they pass through the heat recovery section 26 before the gases pass into the air heater 32, via the duct 30. The gases are mixed with relatively cool air supplied by the forced draft fan 34 in the air heater 32 and the gases, now a mixture of gases and air, exit the air heater through the duct 36. The duct 36 directs the gases into the baghouse 38 where the gases are drawn through the bag filters to separate or remove the very fine solid material from the gases. The separated solid material collected by the filters falls into the hopper sections 50a and 50b and is passed to a waste area (not shown). A portion of the cleaned gases from the baghouse 38 are recycled to the loopseal 28 via the branch duct 42, the forced draft fan 44, and the ducts 46 and 48. The gases are used to assist in passing the solid particulate material from the cyclone separator 22 through the loopseal 28. As a result of the combustion in the furnace section 12, the gases contain approximately 3-7% oxygen, which allows the gas and particulate material mixture to flow through the loopseal 28 without the particulate material oxidizing or burning, thus avoiding the problems set forth above. Water is passed through the tubes forming the walls of the enclosure 12a and the heat exchange tubes in the heat recovery section 26 to extract heat from the particulate material in the enclosure 12a and from the gases in the heat recovery section 26, to progressively convert the water to steam. It is understood that flow circuitry can be provided as necessary to promote the fluid flow. The fluidized bed reactor need not be of the "circulating" type but could be any other type of fluidized bed in which the recycling of the solids increases the efficiency of the overall system.

Claims: A fluidized bed steam generation system comprising furnace (12) for receiving a fluidized bed of particulate material including fuel and for combusting the fuel to form a mixture of entrained particulate material and gases, a separator (22) for separating a first portion of the entrained particulate material from the gases, connecting means (28) for connecting the separator (22) with the furnace (12) to pass the first portion of separated particulate material from the separator (22) to the furnace (12), heat recovery means (26) for passing the separated gases in contact with a cooling media, and a baghouse (38) for receiving the gases from the separator means (22) and for separating a second portion of particulate material from the gases, characterised by an air heater (32) disposed downstream of the heat recovery means (26) and upstream of the baghouse (38) for adding relatively cool air to the cooled separated gases, and means (42, 46, 48) for passing at least a portion of the separated gases back into the connecting means (28) to assist in passing the first portion of separated particulate material through the connecting means (28) and back into the furnace (12). A system as claimed in Claim 1 in which the said portion of the separated gases is passed from the outlet from the baghouse (38) to the connecting means (28). A system as claimed in either preceding claim further comprising means for passing water in a heat exchange relation to the particulate material in the furnace (12) and to the separated gases in the heat recovery means (26) for adding heat to the water and converting it to steam. A system as claimed in Claim 3 in which the means for passing water include a plurality of water tubes forming at least a portion of the walls of the furnace (12). A system as claimed in any preceding claim in which a forced draft fan (34) adds air to the air heater (32). A system as claimed in any preceding claim in which the connecting means (28) include a loopseal (28) for directing the particulate material from the separator (22) to the furnace (12). A system as claimed in Claim 6 in which the connecting means (28) further comprises a J-valve to prevent backflow of the particulate material from the furnace (12). A method of generating steam comprising fluidizing a bed of particulate material including fuel in a furnace (12), combusting the fluidized particulate material in the furnace (12) to form a mixture of entrained particulate material and gases, separating a first portion of the entrained particulate material from the gases, passing that first portion of separated particulate material back to the furnace (12), separating a second portion of particulate material from the gases, and cooling the gases after the first step of separating and before the second step of separating, characterised by adding relatively cool air to the cooled gases before the second step of separating, and by passing at least a portion of the separated gases to the first portion of separated particulate material to assist in passing the first portion of separated particulate material back to the furnace section. A method as claimed in Claim 8 in which the said portion of the separated gases is taken after the second step of separating. A method as claimed in Claim 8 or Claim 9 in which water is passed in heat exchange with the particulate material in the furnace (12) and to cool the separated gases after the first step of separating, to heat the water and convert it to steam. A method as claimed in any of claims 8 to 10 in which the air is added by a forced draft fan (34).

Inventors: TANG JOHN TSIN-YEE, TANG, JOHN TSIN-YEE
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1994
Main classification: F23C11
IPC codes: F23C9, F23C10, F22B31, C10J3